Mystery plant in the backyard

Do you like plant mysteries? If so, I have a good one for you. In recent weeks I've noticed this volunteer in the backyard:

When it was smaller, I thought it was a weed but out of curiosity I decided to let it grow. However, as it got taller, I became even more baffled. 

Actually, there are two of volunteers. They're growing under and next to our compost tumbler. It sat there for several years, with some compacted, dehydrated and semi-composted plant matter inside. The tumbler—or rather the composting process—never really worked the way we had thought, probably because we didn't put enough effort into doing it right. A few months ago I decided to open the tumbler and spray the "compost" with water in an attempt to rehydrate it for possible use in the garden. And promptly forgot about it.

So that's the backstory. But I still didn't know what these mystery plants were. 

I decided to try a couple of plant identification apps I had recently installed on my phone. These apps take a picture of the plant in question and try to match it against their online database.

The first one, Plantsnap,yielded these results:

Hmmm, good try but no go although castor bean looked to be the closest in terms of leaf shape.

I repeated the same exercise in an app called Garden Answers. Like Plantsnap, it suggested castor bean and tetrapanax (as if!) as well as things like fatsia, Japanese maple and fig. No match.

After some deep thinking (a nap was involved) I decided to post a photo to the Plant Identification group on Facebook. Membership is open to anybody, and with over 133,000 members around the world, there's a vast pool of knowledge to be tapped into.

Within minutes (literally!), I had the answer: Carica papaya.

I had a hard time believing it at first, but looking at photos of papaya leaves on Google Images confirmed the ID.

But where did the seeds for these papayas came from? My best guess is that they must have been in the old compost in the tumbler. Either they fell out when I was messing with it, or—more likely—rats dragged some of the compost out of the tumblr, and the papaya seeds along with it.

Unfortunately, papayas are tropical and can't take any frost. Unless we have an exceptionally mild winter, our two volunteers probably won't be around in a few months. For now I'm leaving them to see what happens. The compost tumbler will be Freecyled (our kitchen scraps go into our city-provided green bin) and I will finally be able to plant the Calistemon 'Slim' I bought at the first UC Davis Arboretum plant sale in early October.

For fun, here are some papaya photos from our 2013 trip to Maui, Hawaii where papayas thrive:

In my quick research, I learned that, technically speaking, the papaya is an herbaceous plant, not a tree, because it never forms real wood. Carica papaya is native to Mexico and Central America. Called "fruit of the angels" by Christopher Columbus, it's one of the healthiest fruits in the world.